This invention relates generally to electrodialysis of a salt solution for generating acid and base solutions and, more particularly, to an improved method and four channel bipolar membrane stack for producing acid and monovalent base from untreated, impure salt.
It is well known in the art that acid and base solutions may be generated from a salt solution by the electrodialysis water splitting process. In general, the electrodialysis process consists of a stack containing a plurality of cation-selective membranes, bipolar membranes, and anion-selective membranes positioned between a pair of electrodes. The stack itself comprises an assembly of a plurality of unit cells, each unit cell comprising the said membranes arranged to provide a plurality of parallel flow paths or channels therebetween. When a direct electrical current is applied to the bipolar membrane, water is split into hydroxyl ions and hydrogen ions which migrate or travel to the anode and cathode respectively. At the same time, the cation-selective membrane passes cations traveling toward the cathode while blocking anions, and the anion-selective membrane passes anions traveling toward the anion while blocking cations. If a salt solution such as sodium chloride is directed through the channel between the cation and anion-selective membranes, the concentration of that solution is depleted as hydrochloric acid and sodium hydroxide are formed in adjacent acid and base containing channels. With suitable controls, pumps and associated collecting vessels, the acid and base may be withdrawn for subsequent use and the concentration of the salt solution replenished as required.
An electrodialysis process of the type indicated is fully described in the commonly assigned U.S. Pat. No. 4,880,513, and the teachings of that patent are incorporated herein by reference. The method disclosed in said patent is a fill and draw or batch method wherein various controls, such as pH and liquid level switches are utilized to periodically withdraw the desired acid and base and replenish the raw material salt solution. That process also utilizes a common bipolar membrane stack consisting of three-channel unit cells comprising a bipolar membrane having an anion-selective membrane on one side thereof and a cation-selective membrane on the opposite side thereof.
The production of acid and monovalent base, such as hydrochloric acid and sodium hydroxide or sulfuric acid and potassium hydroxide, in an electrochemical cell requires the raw material salt (i.e., sodium, chloride or potassium sulfate) to be virtually free of multi-valent cations like calcium, magnesium and iron. Calcium and magnesium will precipitate in the cation-exchange membrane of these cells in a high pH environment leading to irreversible damage to the cation-exchange membrane. This problem is inherent in the production of chlorine and caustic in an electrochemical cell and, more particularly, in the production of hydrochloric acid and sodium hydroxide in the typical three-channel bipolar membrane stack.
A raw material salt, like sodium chloride, is most frequently contaminated with salts of calcium and magnesium. Similarly, raw material potassium sulfate is contaminated generally with iron salts. In an effort to eliminate the problem, current practice dictates that sodium chloride brines be pre-treated, by precipitation processes and ion exchange, to remove calcium and magnesium ions. Such methods are expensive and time-consuming, requiring additional equipment, chemicals and manpower to rid the brine of the unwanted impurities. In addition, a waste stream is created by the pre-treatment methods that requires treatment prior to disposal. The same type of pre-treatment and problems are inherent in the case of raw potassium sulfate contaminated with iron.
There thus exists a need for a method and apparatus that permits the production of sodium hydroxide or potassium hydroxide from raw and untreated salts containing multivalent cations like calcium, magnesium or iron, which would normally precipitate within a cation-exchange membrane in a high pH environment, eventually damaging said membrane.